Inkjet printing mechanisms may be used in a variety of different products, such as plotters, facsimile machines and inkjet printers, collectively called in the following as printers, to print images using a colorant, referred to generally herein as "ink." These inkjet printing mechanisms use inkjet cartridges, often called "pens," to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with an entire supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as "off-axis" systems, propel only a small ink supply with the printhead carriage across the printzone, and store the main ink supply in a stationary reservoir, which is located "off-axis" from the path of printhead travel. Typically, a flexible conduit or tubing is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a "pen."
Each pen has a printhead that includes very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and substantially perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The height of this band is known as the "swath height" of the pen, the maximum pattern of ink which can be laid down in a single pass.
The orifice plate of the printhead, tends to pick up contaminants, such as paper dust, and the like, during the printing process. Such contaminants adhere to the orifice plate either because of the presence of ink on the printhead, or because of electrostatic charges. In addition, excess dried ink can accumulate around the printhead. The accumulation of either ink or other contaminants can impair the quality of the output by interfering with the proper application of ink to the printing medium. In addition, if colour pens are used, each printhead may have different nozzles which each expel different colours. If ink accumulates on the orifice plate, mixing of different coloured inks (cross-contamination) can result during use. If colours are mixed on the orifice plate, the quality of the resulting printed product can be affected. For these reasons, it is desirable to clear the printhead orifice plate of such contaminants and ink on a routine basis to prevent the build up thereof. Furthermore, the nozzles of an ink-jet printer can clog, particularly if the pens are left uncapped in an office environment.
In an off-axis pen, the life goal is on the order of 40 times greater than a conventional non off-axis system, e.g. the printhead cartridges available in DesignJet.COPYRGT. 750C color printers, produced by Hewlett-Packard Company, of Palo Alto, Calif., the present assignee. Living longer and firing more drops of ink means that there are greater probability that the printer print quality degrade and/or deviate along life. This requires finding better ways to keep functional and stable our printheads during long periods and large volumes of ink fired.
In U.S. Pat. No. 5,455,608 it is described how a printer may adjusts servicing of the pen based on the result of the current drop detection step only. Before starting a plot these printers perform a drop detection on all the pens to detect if there are any non-firing nozzles ("nozzles out"). If a single nozzle out is detected in a pen, the printer triggers a so called automatic recovery servicing process for servicing the malfunctioning pen to recover the malfunctioning nozzle(s).
This process includes a sequence of 3 nozzle servicing or clearing procedures of increasing severity which are performed in sequence so long as some of the nozzles of the printhead fail to fire ink drops pursuant to ink firing pulses provided to the printhead or until all of the procedures have been performed.
At the end of each of these procedures a new drop detection is performed on the pen, to verify if the pen is fully recovered. If, according to the current result of the drop detection, it is not, the subsequent servicing procedure is performed. If, at the end of the 3 functions, the pen is still not fully recovered (i.e. at least one nozzles is still out) the user is reported to replace the pen or to disable the nozzle check. One big drawback of this system when implemented, e.g. as in DesignJet.COPYRGT. 750 C printers, is that if the printer is not able to fully recover the failing nozzles or there are some unstable nozzles, the system will remain in this recovery servicing mode until the decease of the printhead, being forced, by the permanent nozzle out, to run this process at the beginning of each plot. This usually leads to either an unacceptable loss of throughput and printer productivity (because the printer stops and waits for an answer, the automatic recovery process is very time consuming, and causes a big loss of ink particularly when running the priming functions) or to excessive printhead replace or continue messages that users disable nozzle check via front panel, causing throughput losses.
It is known to use error hiding to improve the print quality. In EP patent application no. 98301559.5 it is described a technique which uses a pattern based nozzle health detection technique, based on a LED line sensor mounted on the pen carriage which reads a printed pattern to find misdirected or missing dots corresponding to nozzles out, weak and some kinds of misdirection.
This technique is executed each certain number of plots and apply error hiding on the failing nozzles. However, this approach has some limitations:
It is slow and this limits the number of times that it is possible to perform without heavily affecting throughput and printer productivity. This means that the result of a single detection will be used for several plots with the risk of printhead nozzle health changing over time. PA1 Only the most recent detection is used, making impossible adjusting the error hiding strategy to printhead nozzle health dynamic variations, such as internal contaminants moving inside the nozzles, air accumulation, nozzle plate dirtiness, head crashes (printhead touching media while printing), external contaminants moving on the nozzle plate, or the like. PA1 Each cycle of the technique implies a certain waste of media or a media change since cannot successfully work on all media. PA1 b being a weighting factor; Dnozz[i] being the content of the history for said ink ejection element, as a series of historical values representing the health of the ink ejection element; and n being the number of historical values to be kept into account for said ink ejection element.
With reference to the present application with the term plot it is identified any kind and size of printed output of the printer, seen by the printer as a single job. The plot could then identifies a CDA image or a graphic image like a photo or any other kind of print.
In order to maintain the quality of the printed output of the printer device it is important to improve the certainty that each instruction to the printhead to produce an ink drop from a nozzle of the plurality of nozzles does will produce such an ink drop.